So.. #canon states that #Applejack is at least 16.5, likely around 18 years old. She was able to speak at her first reunion (the Cakes couldn't at 1 month), and she appeared fully or near fully grown in another reunion photo. Each reunion is 100 [full] moons apart (8 years and 30 days), so, assuming the most recent is AJ's 3rd reunion, that gives her a fairly definitive age keeping in mind that Faust said the mane 6 were teens.